The invention relates generally to comparators and, more particularly, to a comparator comprising a differential amplifier stage comprising two emitter-interconnected bipolar transistors whose bases form the two inputs of the comparator, a first load element connected to the collector of the first bipolar transistor and a second load element connected to the collector of the second bipolar transistor.
Comparators based on differential amplifiers including bipolar transistors have a wealth of applications. In some applications it is important that the switching time of the comparator does not exceed a critical value so as not to sacrifice the advantages of the bipolar transistor, namely the low offset voltage of the differential amplifier stage and high gain.
An object of the invention is to configure a comparator of the aforementioned kind so that it features a particularly fast response.
This is achieved for a comparator of the aforementioned kind in that the comparator comprises in addition a first MOSFET and a second MOSFET, a circuit point at which the drop in voltage across the first load element in operation of the differential amplifier stage occurs is connected to the source of the first MOSFET and the backgate of the second MOSFET, the circuit point at which the drop in voltage across the second load element in operation of the differential amplifier stage being connected to the source of the second MOSFET and the backgate of the first MOSFET, and the drain of the second MOSFET forming the output of the comparator.
The comparator in accordance with the invention achieves a very fast response in that the output signal of the differential amplifier configured with high-gain bipolar transistors is used to control a MOSFET at the output which is activated by this signal both via the gate and the backgate, i.e. the substrate of the MOSFET. By employing MOSFETs at the output in conjunction with the backgate activation as described a very low current consumption of the comparator is also achieved.